Process and apparatus for heating titanium tetrachloride vapor

ABSTRACT

PROCESS AND APPARATUS FOR HEATING TITANIUM TETRACHLORIDE VAPOR TO TEMPERATURES AT WHICH IT MAY BE UTILIZED DIRECTLY IN VAPOR-PHASE OXIDATION TO FORM TITANIUM DIOXIDE. THE VAPOR IS HEATED BY PASSING IT SUCCESSIVELY THROUGH AN INLET CONDUIT, A PLURALITY OF HEATING TUBES ARRANGED IN PARALLEL   RELATIONSHIP WITH ONE ANOTHER AND AN OUTLET CONDUIT. THE INLET AND OUTLET CONDUITS AND THE HEATING TUBES ARE CONSTRUCTED OF PLATINUM OR AN ALLOY OF PLATINUM WITH RHODIUM, RUTHENIUM OR IRIDIUM.

`4 Jian. 12, 1971 y P, M, JQNES ETAL PROCESS AND `APPARATUS FOR HEATING TITANIUM TETRACHLORIDE VAPOR y lFiled March 1s, 196e lll PETER MAURILE lIONES DFNNIS READ United States Patent O 3,554,708 PROCESS AND APPARATUS FOR HEATING TITANIUM TETRACHLORIDE VAPOR Peter M. Jones, Keelby, and Dennis Read, Laceby, England, assignors to Laporte Titanium Limited, London, England, a British company y Filed Mar. 13, 1968, Ser. No. 712,769 Claims priority, application Great Britain, Mar. 29, 1967, 14,220/ 67 Int. Cl. C01l13/14 U.S. Cl. 23-293 12 Claims ABSTRACT F THE DISCLOSURE 'Process and apparatus for heating titanium tetrachloride vapor to temperatures at which it may be utilized directly in vapor-phase oxidation to form titanium dioxide. The vapor is heated by passing it successively through an inlt conduit, a plurality of heating tubes arranged in parallel relationship with one another and an outlet conduit. The inlet and outlet conduits and the heating tubes are constructed of platinum or an alloy of platinum with rhodium, rutheniurn or iridium.

BACKGROUND OF THE INVENTION It is known to prepare titanium dioxide by the vaporphase reaction of titanium tetrachloride with oxygen or oxygen containing gases. Such reactions are effected at relatively high temperatures which necessitate the preheating of the titanium tetrachloride reactant component to temperatures ofat least about 1000 C. Heretofore, various processes and forms of apparatus have been proposed for effectively heating vaporous titanium tetrachloride to such temperatures.

SUMMARY OF THE INVENTION The present invention provides apparatus for heating titanium tetrachloride vapor which comprises an inlet conduit, an outlet conduit and a plurality of tubes arranged in parallel relationship and in communication with the said conduits. Means also is provided for heating the tubes. The inlet and outlet conduits and the tubes are constructed of platinum or an alloy of platinum with rhodium, ruthenium or iridium.

The invention also provides a process for heating titanium tetrachloride Vapor which comprises fiowing the vapor from an inlet conduit to an outlet conduit through a plurality of tubes arranged in parallel with one another while heating the tube's.

The titanium tetrachloride vapor may be heated either alone or in admixture with the vapor of one or more other chlorides, for example, aluminum chloride.

The inlet and outlet conduits and the tubes are preferably constructed from a platinum-rhodium alloy containing rhodium within the range of from 5% to 25% by weight, preferably, within the range of from 10% to 15% by Weight, based on the Weight of the alloy. If platinumportion of ruthenium or iridium, respectively, s advan- ICC tageously within the range of from 5% to 25 (preferably, 10% to 15 by weight, based on the weight of the alloy.

The use of platinum or a platinum alloy as the material of construction makes it possible to heat the titanium tetrachloride Vapor to temperatures substantially in excess of 500 C., which is about the maximum temperature that can be satisfactorily achieved using other metals such as, for example, nickel.

When preheating titanium tetrachloride vapor prior to its vapor-phase oxidation in the manufacture of titanium dioxide, it will usually be found necessary to heat the tubes to a temperature of around 1000 C. and, as at that temperature radiation plays an important part in the transfer of heat to the tubes when the tubes are heated externally, the outer surfaces of the tubes prefera-` bly are treated to increase their absorptivity if the tubes are to be heated externally. The tubes may be treated for this purpose by the electrolytic deposition of platinum black upon them and in this Way the emissivity (which is, of course, proportional to the absorptivity at any given temperature) can be increased, in the case of platinum, from the Value of 0.17 for polished platinum to about 0.6, but it falls to about 0.4 if the platinum is subjected to prolonged heating. Preferably, the tubes are treated for this purpose by roughening their outer surfaces with a file or an emery cloth. In this Way, the emissivity of platinum can again be increased to about 0.6 and this value is maintained even after prolonged heating. Closely similar results are obtained when tubes constructed of platinumrhodium or platinum-ruthenium or platinum-iridium alloys are treated by the deposition of platinum black or by roughening the outer surfaces of the tubes as hereinbefore mentioned. When the tubes are heated by passing an electric current through them, it is of course desirable that the emissivity of the outer surfaces of the tubes be as low as possible and therefore these surfaces are then preferably polished.

The tubes may be heated by passing a hot gas over them, but they are preferably heated by means of an electric furnace within which the tubes are situated. Instead of heating the tubes externally, each tube may be heated by passing an electric current through it. The use of electric heating, Whether effected externally or by passing an electric current through the tubes, has the advantage, as comparedwith the use of a hot gas stream, of avoiding the risk that the tubes may be attacked by sulphur frequently contained in the hot gas stream or as a result of reducing conditions obtained in the hot gas stream. Precautions should, of course, be taken to prevent air from entering the apparatus with the titanium tetrachloride vapor to keep corrosion to a minimum. For the purpose of heating titanium tetrachloride vapor prior to its vapor-phase oxidation to titanium dioxide, it is not usually necessary to heat the tubes to a temperature exceeding 1100 C.

The configuration of the tubes and the method of supporting them must, of course, take into account the thermal expansion and contraction that occur in the tubes during operation. It is generally desirable to arrange that the conduits be held lixed Without undue thermal stresses being set up in the apparatus. Thus, each tube may be clamped adjacent to the outlet conduit and connected to the inlet conduit by means of a flexible bellows, the conguration of the tube being such that thermal expansion and o contraction of the tube in operation results in movement of the end of the tube that is adjacent to the inlet conduit substantially along the axis of the bellows. Advantageously, however, the tubes are U-shaped and hang from the conduits so that they are free to expand and contract in a vertical direction.

The internal diameter of each tube may be Within the range of from da inch to 1 inch. The optimum thickness of the wall of each tube depends to some extent on the internal diameter of the tube, but it is usually preferable to use a thickness within the range of from 0.01 inch to 0.05 inch. When the internal diameter of each tube is 0.5 inch. the wall thickness is preferably about 0.02 inch.

It is usually desirable to ensure that the rate of How of the titanium tetrachloride vapor is substantially the same through each tube and, if there are substantial pressure gradients within the conduits, uniform distribution between the tubes requires either that the pressured drop across each tube be large by comparison with the pressure drops within the conduits between the ends of the tubes or that such pressure drops within the conduits be compensated for by arranging that different tubes have different impedances, for example, by fitting the tubes with orifices of different sizes.

The invention also provides a process and apparatus for the manufacture of titanium dioxide by the vapor-phase oxidation of titanium tetrachloride wherein the titanium tetrachloride is preheated by apparatus comprising an inlet conduit, an outlet conduit and a plurality of tubes connected in parallel between the said conduits as hereinbefore specified, the inlet and outlet conduits and the said tubes being constructed of platinum or an alloy of platinum with rhodium, ruthenium or iridium.

For this purpose, titanium tetrachloride vapor may be supplied to the inlet conduit at a temperature of about 160 C. and heated in the tubes to a temperature of about 1000 C. To preheat the titanium tetrachloride to a ternperature of 1000 C., the tubes must be heated to a temperature approaching 1100 C. and when the tubes are heated by situating them within an electric furnace, the temperature of the furnace walls will normally need to be maintained at a temperature within the range of from 1200 C. to 1350 C.

One form of apparatus suitable for preheating titanium tetrachloride vapor prior to its vapor-phase oxidation in the manufacture of titanium dioxide and constructed in accordance with the invention will now be described by way of example in greater detail with reference to the accompanying drawing, in which:

FIG. 1 is an end elevation view of the apparatus;

FIG. 2 is a side elevation view of the apparatus; and

FIG. 3 is a plan view of the apparatus.

Referring to the accompanying drawing, the apparatus comprises a straight inlet conduit 1 mounted with its axis horizontal and a straight outlet conduit 2 mounted with its axis parallel to, and lying in the same horizontal plane as, the axis of the inlet conduit 1. Each of the conduits 1 and 2 is formed with a row of approximately equally spaced apertures in its underside. The number of apertures in the conduits 1 and 2 are the same and the arrangement is such that, for each of the apertures in one of the conduits, a vertical plane at right angles to the axis of the conduit passing through the center of the aperture also passes through the center of an aperture in the other conduit.

The conduits 1 and 2 are interconnected by a plurality of U-tubes 3, which are connected in parallel and which hang vertically downwards from the conduits and are each so shaped that the height of the tube is much greater than the horizontal separation between the limbs. One end of each tube 3 is in register with an aperture in the conduit 1, and the other end of the tube is in register with the adjacent aperture in the conduit 2, thus providing communication between the conduits 1 and 2. The axes of the limbs of each U-tube 3 lie in a vertical plane that Cal i is at right angles to the axes of the conduits 1 and 2. The conduits 1 and 2 and the tubes 3 are constructed of a platinum-rhodium alloy having a rhodium content of 10% by weight based on the weight of the alloy.

The external surfaces of the tubes 3 are roughened by tiling to increase the emissivity of these surfaces to about 0.6.

The conduits 1 and 2 are each supported from above by members 4 which support each conduit at points spaced apart along the length thereof. The portion of each member 4 that passes underneath the conduit 1 or 2 that it is supporting is concave upwardly and is provided on its upper surface with a layer of ceramic refractory material 5 (for example, sintered alumina) upon which the conduit rests.

Because of the configuration of the U-tubes 3, much the greater part of the thermal expansion and contraction that occur during operation are in a vertical direction, in which movement is unrestrained. The small amount of expansion and contraction that occur in a horizontal direction are readily taken up by slight deformation of the U-tubes 3 without large stresses being set up. The manner in which the conduits 1 and 2 are mounted is such that they can expand and contract axially against the friction between the conduits and the ceramic material 5.

The apparatus is adapted for use in an electric furnace having a lining of a refractory material and provided with U-shaped silicon carbide resistance elements.

In operation, the titanium tetrachloride vapor to be heated is supplied to the inlet conduit 1 through the open end 6 thereof and then through the U-tubes 3, where it is heated, to the outlet conduit 2 of which the end 7 is open, the pressure drop across the U-tubes being suiciently high to ensure that the rate of flow of the titanium tetrachloride is, when the apparatus has the dimensions hereinbefore stated to be suitable, about pounds per hour through each U-tube.

While the invention has been described with respect to what at present are considered to be preferred embodiments thereof, it will be understood, of course, that certain changes and modications may be made therein without departing from its true scope as dened by the appended claims.

What is claimed is:

1. Apparatus for heating titanium tetrachloride vapor which comprises an inlet conduit, an outlet conduit, and a plurality of tubes arranged in parallel relationship with one another and in open communication between the said conduits, and means for heating the tubes, said inlet and outlet conduits and said tubes being constructed of metals selected from the group consisting of platinum and alloys of platinum with rhodium, ruthenium or iridium.

2. Apparatus as set forth in claim 1, wherein the inlet and outlet conduits and the tubes are constructed from a platinum-rhodium alloy containing rhodium within the range of from 5% to 25% by weight based on the weight of the alloy.

3. Apparatus as set forth in claim 2, wherein the platinum-rhodium alloy contains rhodium within the range of from 10% to 15% by weight based on the weight of the alloy.

4. Apparatus as set forth in claim 1, wherein each tube is clamped adjacent to the outlet conduit and connected to the inlet conduit by flexible bellow means.

S. A process for heating titanium tetrachloride vapor which comprises flowing said vapor from an inlet conduit to an outlet conduit through a plurality of tubes arranged in parallel relationship with one another while heating said tubes, the inlet and outlet conduits and said tubes being constructed of a metal selected from the group consisting of platinum and alloys of platinum with rhodium, ruthenium or iridium.

6. A process as set forth in claim 5, wherein the titanium tetrachloride vapor is heated in admixture with the vapor of at least one other chloride.

7. A process as set forth in claim 6, wherein the titanium tetrachloride vapor is heated in adrnixture with aluminum chloride vapor.

8. A process as set forth in claim 5, wherein the tubes are heated externally.

9. A process as set forth in claim '5, wherein the tubes are heated by contacting their outer surfaces with a hot gas.

10. A process as set forth in claim S, wherein the tubes are heated by means of an electric furnace within which the tubes are situated.

11. A process as set forth in claim 5, wherein each tube is heated by passing an electric current through it.

12. A process as set forth in claim S, wherein the titani- 6 a temperature of about 160 C. and heated in the tubes to a temperature of about 1000n C.

References Cited UNITED STATES PATENTS 1,917,042 7/1933 Lewis 165-176 2,164,605 7/1939 Young 165--81 2,463,396 3/1949 Krchma 20-3-86 3,443,630 5/1969 Auld 165-1 0 NORMAN YUDKOFF, Primary Examiner SAM SILVERBERG, Assistant Examiner U.S. Cl. X.R.

um tetrachloride vapor is supplied to the inlet conduit at 15 23 02y 252; 106 300 165 81; 203 .g7 

